Assessing the Crustal Stratigraphy of Mercury: Results from MESSENGER Orbital Observations

Mariner 10 and MESSENGER flyby images revealed subtle color variations across Mercury's surface, many of which are associated with impact craters and basins. Impact craters that excavated material spectrally distinct from the surrounding pre-impact surface serve as windows into the subsurface, allowing observations of material at depth that would otherwise remain hidden to remote observations. Previous localized studies of spectrally distinct, excavated material suggested the presence of buried volcanic plains and a heterogeneous crustal stratigraphy and support the important role of volcanism in the evolution of the crust. MESSENGER's Mercury Dual Imaging System (MDIS) has since acquired global color (1 km/pixel) and high-resolution monochrome (250 m/pixel) base maps of the innermost planet, taken under illumination and observation geometries optimized for observing both color and morphology. These base maps, along with detailed targeted observations (up to 80 m/pixel color and 10 m/pixel monochrome), allow detailed co-mapping of geologic and spectral features across Mercury's surface. Such mapping, along with the use of scaling laws and melt-volume calculations to bound the depth of origin of crater ejecta and central peak structures, allows us to investigate many aspects of Mercury's crustal stratigraphy, including, but not limited to, the following topics: (1) The stratigraphy of four of Mercury's younger large basins: Caloris (1550 km in diameter), Rembrandt (720 km), Beethoven (630 km), and Tolstoj (360 km), which have all been flooded by spectrally distinct volcanic plains. Post-flooding craters of varying sizes enable an estimation of the thickness of the volcanic fill and the nature of the pre-flooding basin floor. These measurements will help to constrain models for subsequent compensation, uplift, and deformation; and incorporation of topography and gravity data link basin fill with the broader lithospheric evolution of Mercury. (2) The depth of origin and distribution of low-reflectance material (LRM), a likely compositional end-member. LRM centers are often associated with large crater and basin ejecta, implying that this material must originate at substantial depth. In previous studies, LRM was found to be the color unit with the greatest depth of origin. A more complete knowledge of LRM distribution in the crust will enable a better assessment of the nature of the material's pre-excavation origin. (3) The location and thickness of pre-Tolstojan (> 4.0 Ga) volcanic plains. Such plains tend to be located beneath younger intermediate plains and may be used to assess the extent of an older generation of volcanic activity.